This invention relates to sinusoidal wave oscillator ballast circuits and especially to such circuitry suitable for use with fluorescent lamps of the 35 to 40 watt variety.
Presently manufactured ballast circuits for fluorescent lamps are, most frequently, of the 120 Hz auto-transformer type. Therein, the saturation characteristic of the transformer is employed to provide the desired currents necessary to the operation of a fluorescent lamp.
However, the auto-transformer type of ballast is known to be relatively heavy and cumbersome. Also, it is known that such apparatus is relatively inefficient which leads to excessive heat generation as well as energy loss. Moreover, the operational capabilities are something less than desired in view of the relatively low operational frequency of 120 Hz which is well within the audible range.
Another known form of lamp ballast circuitry employs a flip-flop type oscillator circuit in cooperation with a saturable core transformer. A transistor of the oscillator saturates and effects saturation of the core material of the transformer to limit current flow and inhibit lamp burnout. However, core material saturation characteristics are relatively erratic and unpredictable which renders such circuitry undesirable or at best, most difficult to accurately predict or control.
In still another form of lamp ballast circuitry, a rectangular-shaped waveform is developed and applied to a filter network. Therein, the rectangular waveform is converted to a sinusoidal waveform. However, rectangular-shaped waveform circuitry has been found less efficient than circuitry wherein a sinusoidal waveform is developed directly. Also, the required filtering to provide a sinusoidal waveform derived from a developed rectangular-shaped waveform is undesirably expensive.
A further form of lamp ballast apparatus is set forth in the previously-mentioned application entitled "Tuned Oscillator Ballast Circuit" filed in the name of the present inventor. As mentioned, the circuitry relates to a tuned oscillator having a plurality of inductive windings to effect development of resonance at a given frequency, activation of the oscillator coupled to a lamp circuit, and clamping of the circuitry to inhibit uncontrolled current flow through the oscillator.
Additionally, the prior art provided separate circuits for both transient signals and radio frequency interference (RFI). Moreover, the known transient filter circuits included either a single or "stacked" transient responsive devices while the RFI circuits included at least two inductors and a bifilar wound transformer. Such circuitry is relatively expensive and appears to leave much to be desired.